A great variety of medical endoprosthesis or implants for various uses are known from the prior art. Endovascular prostheses or other endoprosthesis, for example stents, fastening elements for bones, for example screws, plates, or nails, surgical suture material, intestinal clamps, vascular clips, prostheses in the region of the cardiac and soft tissue, and anchor elements for electrodes, in particular for pacemakers or defibrillators, are understood to be implants within the meaning of the present invention.
Stents which are used for treatment of stenoses (vascular constrictions) are utilized particularly frequently as implants. Stents have a body in the form of an (optionally open-worked) tubular or hollow cylindrical basic mesh which is open at both longitudinal ends. The tubular basic mesh of such an endoprosthesis is inserted into the blood vessel to be treated and is used to support the vessel. Such stents have become established in particular for the treatment of vascular diseases. Use of stents allows constricted regions in the blood vessels to be expanded, resulting in lumen gain. Although the optimal vessel cross section primarily necessary for successful treatment may be achieved by the use of stents or other implants, the permanent presence of such a foreign body initiates a cascade of microbiological processes which may promote inflammation of the treated blood vessel or a necrotic alteration of the vessel, for example, and which may lead to gradual overgrowth of the stent as the result of plaque formation. In the worst case this alteration of the blood vessel may result in vascular occlusion.
Implants made of a biodegradable material are frequently used at the present time. Such implants have proven to be advantageous with regard to the problems described above.
Suitable materials for the body of biodegradable implants may contain polymers or metals, for example. The body may be composed of several of these materials. The common feature of these materials is their biodegradability. Examples of suitable polymeric compounds include polymers from the group including cellulose, collagen, albumin, casein, polysaccharides (PSAC), polylactide (PLA), poly-L-lactide (PLLA), polyglycol (PGA), poly-D,L-lactide-co-glycolide (PDLLA-PGA), polyhydroxybutyric acid (PHB), polyhydroxyvaleric acid (PHV), polyalkyl carbonates, polyortho esters, polyethylene terephtalate (PET), polymalonic acid (PML), polyanhydrides, polyphosphazenes, polyamino acids, and the copolymers thereof, as well as hyaluronic acid. Depending on the desired characteristics, the polymers may be present in pure form, derivatized form, in the form of blends, or as copolymers. Metallic biodegradable materials are primarily based on alloys of magnesium and iron. The present invention preferably relates to implants whose biodegradable material at least partially contains a metal, preferably iron, manganese, and/or tungsten, in particular an iron-based alloy (referred to below as “iron alloy” for short).
Other metallic materials may also be used as implant materials. Implants containing an iron alloy, in particular iron-containing stents, are particularly economical and easy to manufacture.
The term “biodegradation” refers to chemical, in particular hydrolytic, enzymatic, and other metabolic degradation processes in the living organism, which are primarily caused by the bodily fluids which come into contact with the biodegradable material of the implant, resulting in gradual dissolution of the structures of the implant containing the biodegradable material. As a result of this process the implant loses its mechanical integrity at a specific time. The term “biocorrosion” is often used synonymously for “biodegradation.” The term “bioabsorption” includes the subsequent absorption of the degradation products by the living organism.
In the future, it would be desirable to avoid or reduce to the greatest extent possible the above-described inflammation-promoting effect of implants, since this decreases the effectiveness of the implant and may cause further damage to the organism being treated.
U.S. Pat. No. 6,613,432 B2 discloses a plasma treatment in a plasma comprising nitrogen-containing and oxygen-containing molecules for reduction of the inflammatory reaction and for avoidance of restenosis. The plasma treatment is very brief, lasting less than approximately five minutes, for example. However, such a plasma treatment of implants requires complicated equipment and involves a high level of manual effort. Filigreed components such as stents require intricate fine motor skills for the charging procedures which currently are not achievable using automatic handling systems. Furthermore, such plasma treatments result in loading with materials having anti-inflammatory activity only directly at the surface, i.e., to a depth of a few nm. When the subsequent degradation process reaches zones farther in the interior which are not reached by the plasma treatment, there is no longer any anti-inflammatory activity. During the degradation, these interior zones are sometimes already reached very soon after the implant is inserted into the bodily part to be treated, after the time at which further inflammatory reactions are to be suppressed. In addition, it must be assumed that only relatively small quantities of active substances which reduce inflammation can be delivered from a plasma coating to the vessel wall.